On-going and completed work

Achim Morschhauser (Ph.D. since 2010):
We use Mars Global Surveyor Magnetometer data to investigate the magnetic field of Mars. In contrast to Earth, Mars' magnetic field is not a global dipole field and therefore does not possess an active core dynamo. However, as the remanent lithospheric field is relatively strong, a dipole field in which the crust has been magnetized must have been existed somewhen in the past. We will use the information present in the remanent field to investigate the timing and processes which led to the extinction of the Martian core dynamo. Therefore, we will derive our own spherical harmonic model of Mars' lithospheric field using innovative regularization techniques and model the processes which magnetized the crust in a second step.

Hendrik Hansen-Goos (Post-Doc position 2012-2013):
Habitability of a planet is commonly defined through the availability of liquid water on the planetary surface, the obvious motivation for this being the presumed necessity of a solvent for any kind of life to function. As a result, Mars, due to low surface temperatures is located just outside the habitable zone of our solar system. Our present research focuses on the study of physical mechanisms that have the potential to extend the liquid phase of water beyond its occurrence in the bulk phase diagram. These mechanisms include premelting of ice both due to microscopic interfacial forces and curvature effects as well as pre-deliquescence of hygroscopic salts. The associated amounts of liquid water are small as they occur on dimensions on the order of 10nm but they are stable under low temperature and low humidity conditions. Naturally, their relevance is limited to the realm of microorganisms for which it has been shown that metabolic activity correlates with the amount of premelted liquid available. Our work provides arguments for an adjustment of the outer limit of the habitable zone such as to include Mars, as long as we are concerned with microbial life that is not directly exposed to the surface but rather sheltered in cracks and fissures of the bedrock. The findings are consistent with measurements of metabolic activity of extremophile microorganisms under simulated Martian climate conditions, which are performed by our colleagues from the DLR Experimental Planetary Physics group.

Vlada Stamenkovic (Ph.D 2008-2012): "Habitability and Planetary Dynamics"
ESA Fellow in the ESA cooperation program between ESA, DLR and the University of Münster Habitability has been defined as the ability of a planet to sustain liquid water on its surface. Following this definition many scientists have built up complex atmospheric models that calculate surface temperatures and pressures for planets orbiting different types of stars. This surface habitability is also known as the "habitable zone" or the "circumstellar habitable zone" around stars. If we look at the other planets and moons in our own solar system then we will quickly realize that only Earth, and maybe Mars and Venus in the past, were "surface habitable" planets. Planetary climates are fragile and the last two decades of research indicate that surface habitability is a highly complex phenomenon, which might be rare in the universe. Many planets like Mars do nowadays not sustain surface habitability as we know it. Especially Rogue (non-bound, drifting planets) or Pulsar planets do not have at all or do get only small amounts of external heat by their host star to provide any surface habitability. For all those planets, which might compose the majority of all existing terrestrial planets in the universe, surface habitability is not feasible. But can we tell something about the internal, deep subsurface habitability of planets, which we call HADES (Habitability of the Deep Subsurface) in general? We use parameterized 1 D thermal evolution models to relate the evolution of potential subsurface habitats based on water depths and serpentinization to different planetary objects such as Mars or Europa and Titan. We want to understand what factors enhance the probability of creating long-lasting deep subsurface habitats on different planets. Next to this we also want to know how these processes that might create subsurface habitability interact with the dynamics, i.e. the thermal evolution and the tectonic mode of the planet. To do so we firstly have to understand how in general planets (solar system and extrasolar) evolve in time and how this evolution depends on their host star (stellar class and metallicity: Iron, magnesium, silicon, carbon and oxygen to hydrogen ratios), the planet’s mass and the orbital characteristics. We later want to implement the subsurface domains where water is liquid and where serpentinization reactions can occur. Our goal is to create a general dynamical model of subsurface habitability for exoplanets and to study their evolution with time and their connection to planetary dynamics, especially to plate tectonics. In a second step we want to apply our results to Mars to understand if and where potential deep subsurface habitats could have existed or could still persist.

Christian Hüttig (Post-Doc position since 2013):
The work on numerical methods to simulate a planet’s interior provided tools to study important properties for the planet’s thermal and structural evolution. The focus lies on global simulations by studying the flow structure of the mantle and outer core, providing hints and possible effects on composition and features observable on the surface. Most of my work falls in the generic domain of computational fluid dynamics (CFD). Recent projects include:

Thermo-chemical convection in a rapidly rotating fluid: implications for the iron-snow regime in a spherical shell

Initiation of plate-tectonics: towards a realistic global model

Shape properties of thermo-chemical piles on the core-mantle boundary and implications on viscosity

Thomas Ruedas (Post-Doc position since 2013):
2D and 3D models of convection and melting in Mars, Venus, and similar one-plate planets will be combined with models of the mineralogy and the thermoelastic proper ties of the mantle in order to simulate the evolution of these planets since solidification of the mantle 4.4–4.5 billion years ago, and to explain cer tain geological structures on their surface. Issues of particular interest include the stability of their lithospheres, the various volcanic structures on Venus, and the formation of two apparently long-lived volcanic centers on Mars. Central aspects are the effect of phase transformations of mantle minerals on global convection patterns and mantle plumes dynamics, and the effect of trace components (radionuclides and volatiles) and their redistribution through convection, melting, and volcanism. In the early stages of planetary evolution asteroid impacts play a significant role. Therefore their effects, e.g., on the thermal structure of the mantle or on crustal thickness, are taken into account in the models. In view of the limited knowledge of the composition and the mantle-to-core ratio certain model parameters such as the thickness of the mantle, the ratio of magnesium and iron, and the radionuclide and water content will be varied. Furthermore, the influence of surface temperature and, in the case of Mars, the effect of lateral temperature and thickness variations in the lithosphere caused, e.g., by the crustal dichotomy as well as the role of ancient compositional heterogeneities will be explored. In this context parameter combinations will be considered that are not relevant for Mars or Venus but are of general interest, e.g., with respect to exoplanets. The models will yield geophysical and geochemical observables that can be compared with real observations.

Nicola Tosi (Post-Doc position since 2010):
We work on numerical models of Earth's mantle convection that incorporate recent constraints derived from mineral physics. In particular we investigate the dynamical interplay between strongly pressure-dependent thermodynamic properties and the perovskite to post-perovskite phase transition. A first study conducted in Cartesian geometry has shown that a large thermal conductivity near the core-mantle boundary can yield extremely stable large-scale thermo-chemical structures. This can have important consequences for the generation of the dominant degree-2 anomaly in the lower mantle of the Earth and for the dynamics of planets for which the pressure dependence of thermodynamic and transport properties is more pronounced Earth. Currently we are extending this study to a spherical geometry using Gaia, a 3D mantle convection code available at DLR. In order to make our models self-consistent, help would be appreciated in regard to the most appropriate choice of an equation of state from which to derive radial reference distributions of density, thermal expansivity and conductivity.

Tina Rückriemen (Ph.D. since 2010):
One of the most compelling discoveries of the Galileo mission was the detection of a self-sustained magnetic field at Jupiter’s moon Ganymede. In the present project, the thermo-chemical evolution and the magnetic field history of Ganymede will be modelled using parameterized convection, a thermodynamic description of core processes driving the dynamo, and dynamo models of cores with iron snowfall and iron-sulfide flotation. The aim is to understand how parameters such as core composition and mantle rheology would constrain core convection (thermal or compositional) and dynamo action in the core and to come up with an explanation for the magnetic field of Ganymede. At the same time we need to provide an explanation why a field is lacking on Europa. Recent experimental data on the Fe-FeS system suggest that the dynamo in Ganymede may be compositionally driven but that it may function substantially different than the geodynamo. In the latter, an inner core grows from below driving convection in the core and the dynamo. In Ganymede (and in Europa) the core may freeze via precipitation and the fall of Fe snow and/or through the floating of FeS flakes. This may occur in cores sufficiently close to the eutectic composition in sufficiently small planets and satellites. We will discuss the application of the model to other planets such as Mars.

Manuel Schölling (Ph.D 2009 - 2010):
Compositional differences between meteorites have been interpreted as being indicative of wide variations in the degree of differentiation of their parent bodies (planetesimals). Differentiated planetesimals must have undergone (at least partial) melting caused by short lived nuclides 26Al and 60Fe. Thermal models have shown that planetesimals may experience differing degrees of partial melting depending on the onset time of accretion relative to the time of formation of the Ca-Al-rich inclusions (CAIs), the accretion time, and the final size of the planetesimals. Even the presence of a magma ocean for these bodies has been suggested in the case of rapid accretion. These thermal models base upon thermal conduction only and disregard the possibility of convection before a magma ocean develops. In fact, convection in a solid planetesimal is unlikely. However, it may set in for a sufficient amount of partial melt even before the existence of a magma ocean - e.g., melt reduces the viscosity of the material by 3-4 orders of magnitude for 25% of partial melt in suspension. Whether the existence of convection is possible can be roughly estimated with the internally heated Rayleigh number as a function of the layer thickness for different values of viscosity. First results suggests that the interior may actually convect even at small degrees of partial melting.
Considering that convection increases the heat transport in the interior and that the planetesimal will cool faster under these circumstances, the thermal evolution of a planetesimal and the amount of partial melt that can be produced may differ from what is predicted in earlier studies.

Lena Noack (Ph.D. 2008-2012):
The existence of plate tectonics on Earth is well known even by children, but the question of how these plates form and are transported back into the interior is still unanswered. Taking a look at the other terrestrial planets of our solar system makes it even more challenging to answer this question - only the Earth does have plate tectonics. Venus, Mars, Mercury and the Moon nowadays show one large stable plate and are in the so-called stagnant lid regime. In this regime, convection takes place below a non-moving lid; thus convection does not involve surface plates as in the case of plate tectonics. It is, however, debated that Mars may have had moving surface plates in its early evolution and only later turned into the observed present stagnant lid regime. A change between the convecting regimes has been also discussed for Venus. This change could be responsible for the very young surface that was resurfaced around 700 million years ago. In this PhD thesis, I will investigate the conditions for the different convection regimes together with thermal evolution models to gain a better understanding of the associated processes. A 3D spherical convection code (GAIA developed by Christian Hüttig) will be used and modified accordingly. A particular emphasize in this work will be on the surface conditions and the rheology of the convecting mantle, i.e. the flow behavior of the material.

Ana Catalina Plesa (Ph.D 2008-2014):
Melt generation in a planetary mantle is a complex process that has a strong influence on the thermo-chemical evolution of a planet. The generation and recrystallization of melt tends to buffer temperature variations due to the consumption and the release of latent heat, respectively. The existence of partial melt reduces the viscosity and can therefore enhance the efficiency of planetary cooling. Partial melting in the mantle also results in buoyancy sources that can drive flow and cause further melting. However, a reduction of the viscosity is only given as long as all or part of the melt remains in retention with the mantle material. If melt separates from the mantle, the viscosity increases again to that of the solid material. The extraction of partial melt leaves behind a residuum that is more buoyant than its fertile parent material. This process can lead to the formation of a buoyant upper mantle, which can have a stabilizing effect on the mantle dynamics and prevents further the planet from cooling. In addition melt segregation results in a redistribution of radioactive elements. Radioactive heat sources are incompatible elements and enriched in the melt. If the enriched melt rises toward the surface the mantle becomes depleted in heat sources and a crust forms that is enriched in radioactive heat sources. When basaltic melt is extracted from the mantle and segregates to form the surface layer, heat is transported effectively toward the surface.
In the present project, a systematic study of the role of partial melt in a convecting planetary mantle on the heat transport and the flow pattern will be performed with numerical simulations using a 3D spherical convection code. For this purpose, important effects of partial melting will be studied such as the consumption and release of latent heat, the influence of melt on the mantle viscosity, the density changes due to temperature, melt and composition, as well as the redistribution of radioactive heat sources due to crust formation.

Christian Hüttig (Ph.D. 2004 - 2009):
This work presents a method to simulate mantle convection in a spherical shell with fully spatially varying viscosities. The intention behind this numerical model is to run simulations with closer-to-reality parameters and therefore to study the thermal evolution of planetary mantles in great detail. The formulation of the equations of state base on the finite-volume method with the advantage of utilizing fully irregular grids in three and two dimensions, efficiently parallelized for up to 396 CPUs. While being capable of using common regular grids like projected icosahedra and cubed sphere, an irregular grid with the advantage of varying lateral resolution, the spiral grid, was investigated. Basically any set of nodal positions can, after a Voronoi tessellation that forms the necessary cells for the FV formulation, be used as a discrete basis. The model bases on the Cartesian reference frame and utilizes co-located variable arrangement that holds all unknowns at the defined nodal position. To ensure a divergence free velocity field, the SIMPLE method was employed to ensure mass continuity with a correction of pressure. The discretization method is further complete second-order and treated fully implicit, meaning it is capable of solving steady-state solutions with large time-steps while also solving for strongly time-dependent convection with small time-steps. The discretization of the stress tensor is capable to handle viscosity variations of 8 orders of magnitude from cell-to-cell and up to 45 orders of magnitude system wide. The model was validated by a comparison to analytically known solutions as well as to published results. A comparison with a commercial product yielded also satisfying results. Variables of primary interest for these benchmarks were global quantities as the Nusselt number and volume averaged temperature and velocity as well as local quantities such as maxima / minima of mid-shell temperature and velocity. A convergence test with successively refined grids proved the convergence towards a fixed solution. Quantitative measures of three-dimensional spherical mantle convection are sporadic but the here presented simulations vary only a few percent with published results and contributes to further benchmarks. As an application to mantle convection, the simulation was applied to a parameter study of 88 cases to constrain parameterized laws for purely internally heated convection in a spherical shell. The aspect ratio was fixed to 0.55, similar to the estimated value of the earth. The rheology law bases on a linearized Arrhenius law, commonly known as the Frank-Kamenetskii approximation. Three regimes of convection that were explored with bottom-heated convection in previous studies could be identified with purely internally heated convection as well, with the difference of a low-degree-regime that occurs instead of the sluggish regime that occurs in purely bottom-heated convection. This new regime produces long wavelengths in the same parametric range as the sluggish regime. The surface is completely mobile and the transition to the stagnant lid regime is rather abrupt. Present and newly developed indicators for the different regimes were validated to distinguish between them as this is the first parametric study that follows the transition of the regimes with purely internally heated convection in a spherical shell.

Frank Wagner (Ph.D. 2008-2014): "Interior structure, composition, and evolution of terrestrial-type planetary bodies"
The number of discovered extrasolar planets is growing rapidly due to ongoing improvements and increasing precision in detection methods. Although most of the discovered planets are gas giants so far, the frontier to detect low-mass planets is being quickly pushed down and planets with masses below ten Earth-masses have been announced recently. Some of these low-mass exoplanets are expected to be dry and rocky, which classifies them as Earth-like or terrestrial planets. This similarity to the Earth raises the question of habitable, life-sustaining planets outside our solar system. The aim of this PhD thesis is to model numerically the internal structure and the time-dependent evolution of terrestrial planets with the mass of the Moon to ten times the mass of the Earth. We will use those model simulations to obtain scaling laws for important physical and chemical parameters and to study the fundamental properties of a planetary evolution. We also want to gain a better understanding of global processes influencing the habitability of terrestrial planets throughout their histories, such as changing surface conditions, episodic or continuous volcanism, accumulation and outgassing of volatiles, and the perpetuation of a protective, self-generated magnetic field. This work is part of the HGF Alliance "Planetary Evolution and Life".

Wladimir Neumann (Ph.D. 2010-2014):
The recent discovery of remnant magnetizations of angrites and the CV meteorite Allende suggests that magnetic fields were generated in the iron-rich cores of their parent bodies. The observed magnetizations are consistent with magnetic fields lasting for more than 10 Ma after the formation of Ca-Al-rich inclusions (CAIs). Cooling of an internal magma ocean covered by an insulating crust may have been instrumental for the dynamo activity. The core dynamo would have been sustained by thermal convection induced in the core. Although both classes of meteorites are similar in their magnetization, they are likely to have experienced strikingly differing differentiation histories. The magnetized CV-meteorites are generally considered to be relics of metamorphic primordial crusts covering the differentiated interior of planetesimals. These crusts would be insulating and it is likely that eruptive magmatism accompanied by efficient interior cooling through magma heat transport was absent from these parent bodies. Angrites are generally considered to be relics of basaltic crusts and their formation must have invoked melt that was able to rise to the surface and efficiently cool the interior. We propose to study through numerical model calculations the conditions for how long a body with an internal magma ocean can sustain a dynamo. Using 1D thermal evolution models, we want to consider the melting of the interior due to radiogenic heating by short-lived isotopes (26Al and 60Fe) and cooling and crystallization of the magma ocean through the growth of a crust and stagnant lid.